This invention relates to stabilizing halogenated isobutylene-p-methylstyrene copolymers from discoloration, with a minor amount of a weak base. More specifically, the invention relates to preventing discoloration of polymer under process and storage conditions.
Halogenated rubbers are generally prepared by halogenating a rubbery polymer such as isobutylene-isoprene copolymer or isobutylene-p-methylstyrene copolymer in an organic solvent. This produces a halogenated polymer solution containing hydrogen halide as a byproduct. The halide is typically neutralized with an aqueous base and the resulting mixture is washed and precipitated in water to obtain a neutralized halogenated polymer in the form of a wet crumb, typically containing about 50 percent by weight water. The wet crumb is then dewatered in a dewatering extruder, press, mill, dryer or the like to obtain a dewatered crumb usually less than about 10-20 weight percent water. The dewatered crumb is then finished in an extruder or its equivalent with any other additives such as antioxidants, UV stabilizers or the like to obtain a relatively dry halogenated rubber product which is typically pressed into bales for storage and shipment.
Typically, about 1-1.5 weight percent of an additive such as epoxidized soybean oil (ESBO) has been added to the halobutyl polymers during the finishing stage to prevent dehalogenation and discoloration during storage. However, ESBO is relatively expensive and tends to make the halobutyl polymers more tacky and slippery in drying extruder and adversely affect their the cure properties.
Owing to the higher stability of the brominated isobutylene-p-methylstyrene copolymers available under the trade designation EXXPRO(trademark), and the undesirability of additives such as ESBO, additives such as ESBO are typically not used in the EXXPRO(trademark) polymers. However, there are occasionally noted instances of orange discoloration of the EXXPRO(trademark) polymers. This discoloration is the result of minute amounts of iron bromide present in the dried rubber. The discoloration is not present in the wet rubber. The discoloration is reversible as the rubber is made dry or wet.
The thermal instability of most halogenated polymers such as halobutyl and PVC usually starts with deydrohalogenation and usually results in discoloration, reduction of MW and halogen content, some loss of and physical properties. However, the benzyl bromide of the brominated isobutylene-p-methylstyrene copolymers (BrIBMS) does not have a mechanism available to undergo dehydrohalogenation reactions and the observed orange discoloration does not accompany by any MW degradation, bromine loss or physical property change. Consequently, the observed orange discoloration in BrIBMS is not due to dehydrohalogenation as other halogenated polymers. Also the orange discoloration of BrIBMS appears as isolated spots or strips and only occurs in dry polymer during storage. Consequently it is not obvious that the polymer stabilization techniques developed for halobutyl polymers and other halogenated polymers such as PVC in prior arts can prevent the orange discoloration in EXXPRO(trademark).
One way to minimize dehydrohalogenation of halogenated polymers is to use large excess of caustic (NaOH) during neutralization step to neutralize all HBr/HCI by-products from halogenation. This approach did not eliminate the orange spotting and striping of BrIBMS.
Other common approaches are to add acid scavenger, base or other additive/stabilizer during halogenation or neutralization steps. Examples are:
ESBO is commonly added to halobutyl polymers during finishing, but ESBO is expensive and tends to affect drying extruder operation and cure properties.
Roper et. al. (U.S. Pat. No. 4,130,519) teach the use of Ca Stearate (a metal carboxyate) prior to or during halogenation to minimize MW degradation and discoloration of halobutyl polymers resulting from dehydrohalogenation.
Gardiner et. al. (U.S. Pat. No. 5,087,674) describe the use of acid scavengers such as metal oxides, hydroxides and carboxylates in halobutyl polymers prior to or during halogenation step.
Powers et. al. (U.S. Pat. No. 5,286,804) teach the use of wetting agents (such as Ca Stearate) prior to neutralization step to improve the neutralization efficiency of brominated star-branch butyl polymers.
Chaban et. al. (U.S. Pat. No. 2,507,142) describe the use of chemical complexes of inorganic peroxide and metal salts, such as Na and Mg salts of perboric acid, to minimize the MW degradation and discoloration of PVC and other halogenated polymers resulting from dehydrohalogenation. Generally the presence of peroxide is very undesirable because it can react with benzylic bromide on BRIBMS (thus reduces the functional groups available for cure) as well as the backbone and cause MW degradation.
Accordingly, there remains a need to solve the discoloration of the dried EXXPRO(trademark) polymers, due to the presence of minute amounts of iron bromide and to find less expensive additives for the halobutyl polymers which do not affect the tackiness or green strength of the halobutyl polymers.
This inventions is directed toward a method to prevent orange discoloration due to the presence of minute amounts of iron bromide in BrIBMS polymers by adding 100-500 ppm of a weak base such as NaHCO3 as dry powder during the final drying step (add to the final drying extruder). All above cited prior arts (except Chaban""s patent) require the addition of various metal salt additives at either halogenation step or neutralization step where large excess of strong base (NaOH) is present, but that does not prevent the orange discoloration of EXXPRO polymers during storage.
It has been found that halogenated rubber can be effectively stabilized with a small amount of a weak base, preferably NaHCO3 without adversely affecting the curability, tackiness or green strength of the halogenated rubber. By adding the weak base, during the finishing stage, a stabilized halogenated rubber can be obtained which is free from orange spotting and striping and has virtually no adverse change in physical or vulcanizable characteristics.
In one aspect, the present invention provides an improvement in a method for making BrIBMS. The method includes the steps of halogenating preferably brominating a rubbery polymer in an organic solution to form a solution of halogenated rubber and hydrogen halide, neutralizing the hydrogen halide solution with an aqueous alkaline material, stripping the solvent from the neutralized solution to form a wet halogenated rubber crumb, dewatering the crumb to less than about 10-20 weight percent water, and finishing the crumb with one or more stabilizing additives to obtain a halogenated rubber product. The improvement is that the stabilizing additives in the finishing step include from 15 to 10,000 ppm of a weak metal base, preferably an alkali or alkaline earth metal base and most preferably NaHCO3.
In another aspect, the present invention provides a dry, intimate admixture of halogenated rubber and a stabilizing amount less than 1 weight percent of the weak base.
In a further aspect, the present invention provides an intimate admixture of brominated isobutylene-p-methylstyrene copolymer and 15 to 10,000 ppm of a weak base preferably NaHCO3 comprising less than 1 wieght percent water, preferably less than 0.3 weight percent water.
The halogenation of rubbery polymers is well known from, for example, U.S. Pat No. 4,384,072 to Newman et al.; U.S. Pat No. 4,486,575 to Newman et al.; U.S. Pat No. 4,501,859 to Newman et al.; U.S. Pat No. 5,077,345 to Newman et al; U.S. Pat No. 5,286,804 to Powers et al.; U.S. Pat No. 5,087,674 to Gardner et al.; and U.S. Pat No. 4,513,116 to Kowalski et al.; all of which are incorporated herein by reference. The halogenated rubbers according to the present invention include halogenated IBMS preferably brominated isobutylene-p-methylstyrene copolymers.
The base which is added as a stabilizing additive is a weak base, i.e. a base having pK greater than 1, preferably greater than 6. Weak metal bases such as alkali and alkaline earth metal hydroxides, bicarbonates and carbonates are especially preferred. Sodium bicarbonate is exemplary of the preferred weak alkali metal bases.
The base is used in an amount effective to stabilize the halogenated rubber, usually at least 100 ppm, preferably at least 50 ppm, but can be as low as 15 ppm of the rubber. To prevent discoloration of the halogenated rubbers, there is no particular upper limit of the amount of the base which may be used but it will generally be desirable to use 10,000 ppm or less to avoid adversely effecting the physical and chemical properties of the halogenated rubber such as curability, tackiness and green strength. Curable halogenated rubbers preferably contain less than 10,000 ppm of the base additive, more preferably less than 1,000 ppm, and especially less than 500 ppm. In a preferred embodiment, the stabilizing additive is sodium bicarbonate at a level of 100-500 ppm by weight of the halogenated rubber.
The weak base additive is added to the halogenated IBMS in the manufacturing process after the halogenated rubber has been dewatered, preferably to less than 20 percent water by weight, more preferably to less than 10 percent water by weight. The additive is added during the finishing operation in which the dewatered halogenated rubber is further dried to, for example, less than 0.3 weight percent water, in a finishing extruder, mill, dryer or the like.
The additive is generally added in the form of a concentrated solution, for example, a 10 weight percent aqueous sodium bicarbonate solution. The additive could be added in the form of a dry powder, but this is generally more difficult to meter into the polymer. The additive should be added to the polymer after it is dewatered to avoid removing the additive when additional water is removed. Any water that is included in the additive solution is generally removed in the finishing extruder. Generally, the finished polymer contains less than 1 weight percent water, preferably less than 0.3 weight percent water. It is only necessary that the additive be intimately mixed with the polymer, in the finishing extruder.